Molding is a manufacturing process for producing a shaped article by liquefying a plastic or metal material into a mold cavity and cooling and hardening the material to the configuration of the cavity. In overmolding, a previously molded part can be reinserted to a new mold to allow a new molded layer to form around the previously molded part, and becomes integral to the new article. Overmolding can join two different components together without the use of any adhesives or primers. This combination of two different components allows for the creation of strong, structural products with ergonomic comfort and appeasing display.
Typical injection molding requires the pressure to be greater than 200 bar at 250° C. or above because the molding materials have high viscosity and high softening temperature. While robust components are suitable for injection molding with higher pressure, delicate components are unable to withstand the high pressure and high temperature. An alternative to the typical injection molding process is a low-pressure injection molding. The two processes are virtually identical except that the low-pressure injection molding requires 0.5 to 200 bars at 70° C. to 240° C. for the injection.
Illumination or electronic devices contain fragile components and hence, low-pressure injection molding is preferred over the traditional high pressure injection molding. The electronic device comprises components such as LEDs (light emitting diodes), connectors, sensors, capacitors, micro switches, printed circuit boards, cable bundles, transponders, and the like. LEDs are semiconductor diodes that consume little energy (e.g., a voltage of less than 5 volts or a current of less than 20 milliamps), yet emit light with a brightness exceeding that of a much larger incandescent bulb. A LED, whether a lamp type or a surface mount device (SMD) type, typically comprises a LED chip that is encapsulated by an optically clear and thermally stable material into a device for proper wiring, mounting and functioning.
Various LED molding and overmolding materials exist. WO 2010138221 describes two part liquid molding systems of urethanes, silicones, and acrylics. While performances are acceptable for the two part liquid systems, the liquid form normally leads to use of special techniques such as potting or casting to make a silicone body as well as extended curing times (e.g. more than several hours to days) leading to lower productivity. The extended curing times may, in turn, lead to lack of surface uniformity especially with thicker molds and translate to low optical quality in resulting LEDs.
Epoxy resins are also widely utilized as molding and overmolding composition. However, epoxy resins tend to exhibit poor light stability in that they yellow over time following exposure to ultraviolet (UV) light or to elevated thermal conditions (e.g. a temperature in excess of (>) 110 deg. C. for a time >1000 hours). Yellowing, in turn, leads to a reduction in light output from a LED over time. Moreover, curing often occurs over an extended period of time (e.g. three hours) in order to minimize residual stress within an aliquot of cured epoxy resin. “Residual Stress” refers to a tension or compression that exists in a bulk material without application of an external load such as an applied force or displacement of thermal gradient. As residual stress within LED material increases, adverse effects such as dimensional changes or cracking tend to occur over a LED's lifetime usage.
Polyamides are widely utilized in low pressure injection molding due to their low viscosity. While well-suited as a molding and overmolding material, polyamides are typically amber in color, which is undesirable for optical applications.
U.S. Publication No. 2011/0133245 teaches hydrogenated styrene/butadiene triblock polymer composition for LED encapsulant and overmolding applications; however, the viscosity of the composition is too high for low pressure injection molding processes.
WO200954553A2 teaches that acrylic block copolymers have exceptional clarity, low color and stability which may be useful for optical applications. U.S. Pat. No. 6,894,114 to Kato further teaches that acrylic block copolymers may be used for molding articles; however the materials are limited to traditional high pressure injection molding processes.
More often, acrylic block copolymers are combined with tackifiers as pressure sensitive materials. Pressure sensitive materials exhibit “aggressive and permanent tack” or tack at activation temperature (heat-activatable), and have a modulus value of less than 3×10^6 dyne/cm2 at ambient temperature. U.S. Pat. No. 6,734,256 to Everaerts et al., teaches the use of acrylic block and high amounts of tackifier (>28.5%) in order to obtain a hot melt processable adhesive such as pressure-sensitive adhesive (PSA) or heat activatable adhesive composition. Furthermore, U.S. Pat. No. 7,084,209 to Everaerts et al., teaches the use of acrylic block copolymers with high amounts of tackifier for pressure sensitive adhesive tapes.
There is a need in the art for non-tacky molding and overmolding compositions for low pressure injection molding that possess fast throughput and clarity without damaging the delicate electrical or electronic components of the article to be overmolded. The current invention fulfills this need.